Saturday, August 7, 2010

Ageing rocker replaced

UPDATE: The faulty rocker switch on the Wharfedale PM600 was prised open (by depressing two lugs either side of the cover the top came of it) and the cause proper was found; one of the two sprung contacts inside had become misshapen over time and that pole was no longer connecting when the switch was in the 'ON' position.

It had been temporarily lighting up the power LED in the mid-range of its switching motion as that was when the contact was pressed fully home, but as soon as this pressure lessened (or was finally removed) the contact failed.

The separated components of the dismantled rocker switch

The rocker section itself - note the lug on the outer right

The removed spring contacts. Problem: different angles

The main body of the switch into which the spring contacts were fitted. The circular pads on all surfaces were still in good condition - not that it mattered, as the entire switch was replaced anyway (a perfect match was conveniently found in the workshop)

This side profile shot shows how the connections at the rear are actually lugs which are principally designed to be plugged into connectors. This is no major problem, although some care had to be taken whilst re-soldering the wires onto them because (if too much heat gets transferred to the spring contacts inside the switch) this can cause warping - and would've effectively put us right back at the start!

Anyway, the new switch is now fully installed and operation has been returned to normal ...

Gratuitous pounding

Ricky laying down the drums for Changeling

Friday, August 6, 2010

A funked-out EF86

For the past several weeks it's been obvious that one of the valves in my cloned VOX AC30 amplifier had gone microphonic; the tell-tale sound (even before tapping them for confirmation) can be heard in the short video clip just below when the amp is first powered up:


Several other problems also began to occur at practice a couple of weeks ago, involving a 'tinkling' sound and general feedback wails which weren't being caused by interference from the guitar's pickups (or at least not by its usual means anyway). Thankfully both of these issues seemed to have been remedied by simply removing the valves and refitting them, so I'm assuming they stemmed from poor socket connections (?) ... You can see in the following photograph how the valves appear to be sitting slightly uneven, indicating likely bad connections with their associated sockets. I'm guessing vibration can cause them to shake loose over time.

The next photo shows the wonky 12AX7's all nicely re-aligned.

The microphonic valve was verified as being the EF86 at the front end of the amp (sitting far right in the above photograph). A simple test to find the valve responsible is to simply tap the valves individually (using a drum stick or similar implement) whilst the amp is on. The normal result of doing this should just be a gentle tapping sound on the glass of the valve itself. However, if a valve has gone microphonic you'll hear the tapping amplified through the speakers. This is demonstrated in the following clip:


The faulty valve has now been removed and I await in the post a pair of Telefunken EF86's (new old stock) which were purchased through an online auction last weekend, and only now beginning to learn about the inner workings of my amplifier, knowledge is minimal and therefore ongoing, my approach simply being to trial various components to see what works and what doesn't.

If nothing else, the old Telefunken tubes at least look the part ...

Thursday, August 5, 2010

Rocker switch very much not so ...

With our Wharfedale PM600 powered mixer now into its sixth year, the rocker switch on the front panel has finally started to cause problems and over the last couple of practices we've had trouble powering it up due to the switch's connection intermittently failing. Or rather intermittently being maintained ...

With a little experience in electronics, I decided to open it up to check for any obvious issues with regards the wires leading away from the switch itself. Eight screws (four top and four bottom) that held the PM600 together were undone and the inside electronics and casing simply pulled completely out as a convenient single unit.

I then carefully stripped back the necessary heat-shrink with a scapel to get a better look at the wires inside. Of course, there's no particular concern with these connections breaking because there is only ever minimal stress placed upon them.

Which leads us to the conclusion, namely that everything (somewhat unfortunately!) appeared to be in good order, confirming that the fault itself appears to be within the plastic-encased rocker switch itself. And due to the sheer number of times it's been operated over the years, it would make sense for the failure to relate to this component.

How exactly I get inside this I have no idea though, and I'm guessing that the best solution will actually involve replacing it completely anyway. A brand new component would certainly increase it's reliability at any rate ...